Myogenesis in the chick embryo has been extensively studied with respect to the mononucleated myoblast, the precursor to the fused, multinucleated skeletal muscle fiber. Four distinct classes of myoblasts have been identified by clonal analysis. The appearance of one and possibly two classes has been shown to be dependent on the nervous system in vivo. The fate of each myoblast type will be studied by transplantation of clonally pure chick cell aggregates to developing quail embryo legs and subsequent clonal analysis of the transplanted cells to determine if cell-type transitions have occurred. Histological sections of transplant containing quail legs will be studied for evidence of chick cell fusion into myotubes as an alternative fate. The origin of each myoblast type will be studied by transplantation of potential precursor cell cones and, in the case of nerve-dependent myoblast types, an entirely in vitro nerve-myoblast interaction scheme will be used to identify precursors. When the origins and fates of myoblast types are determined, developmental cell lineages will be constructed for myogenic cells. Methods of interference with normal myogenesis not involving the nervous system will be developed as an adjunct to these cell-type transition studies. It will also be determined whether the clonal morphology differences between fast and slow muscle derived myoblasts are motor nerve dependent or are properties inherent to the uninnervated myoblasts.